text: Carbon is distributed throughout the atmosphere, the biosphere, and the oceans; these are referred to collectively as the carbon exchange reservoir, and each component is also referred to individually as a carbon exchange reservoir. The different elements of the carbon exchange reservoir vary in how much carbon they store, and in how long it takes for the  generated by cosmic rays to fully mix with them. This affects the ratio of  to  in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir. The atmosphere, which is where  is generated, contains about 1.9% of the total carbon in the reservoirs, and the  it contains mixes in less than seven years. The ratio of  to  in the atmosphere is taken as the baseline for the other reservoirs: if another reservoir has a lower ratio of  to , it indicates that the carbon is older and hence that either some of the  has decayed, or the reservoir is receiving carbon that is not at the atmospheric baseline. The ocean surface is an example: it contains 2.4% of the carbon in the exchange reservoir, but there is only about 95% as much  as would be expected if the ratio were the same as in the atmosphere. The time it takes for carbon from the atmosphere to mix with the surface ocean is only a few years, but the surface waters also receive water from the deep ocean, which has more than 90% of the carbon in the reservoir. Water in the deep ocean takes about 1,000 years to circulate back through surface waters, and so the surface waters contain a combination of older water, with depleted , and water recently at the surface, with  in equilibrium with the atmosphere.
  Creatures living at the ocean surface have the same  ratios as the water they live in, and as a result of the reduced / ratio, the radiocarbon age of marine life is typically about 400 years. Organisms on land are in closer equilibrium with the atmosphere and have the same / ratio as the atmosphere. These organisms contain about 1.3% of the carbon in the reservoir; sea organisms have a mass of less than 1% of those on land and are not shown in the diagram. Accumulated dead organic matter, of both plants and animals, exceeds the mass of the biosphere by a factor of nearly 3, and since this matter is no longer exchanging carbon with its environment, it has a / ratio lower than that of the biosphere.